What is the lambda function doing in this code? [duplicate]

Question

I have the following code for a greedy implementation of the traveling salesman problem. I can't wrap my head around what exactly the lambda function does in this code.

def distance(p1, p2):
    return ((p1[0] - p2[0])**2 + (p1[1] - p2[1])**2) ** 0.5

def tsp_greedy(points, start=None):
    if start is None:
        start = points[0]
    to_visit = points
    path = [start]
    to_visit.remove(start)
    while to_visit:
        closest = min(to_visit, key=lambda x: distance(path[-1], x))
        path.append(closest)
        to_visit.remove(closest)
    return path

I realize it's creating an anonymous function which x gets passed into. But I'm not sure what is getting passed into this function. What is x?

x is each value in to_visit iterable. min() computes key value for each value, returns value where key function has lowest value. — Łukasz Rogalski
– Łukasz Rogalski, Commented Dec 2, 2016 at 16:29
Does this answer solve your question? stackoverflow.com/a/18296814/406423 — MadMike
– MadMike, Commented Dec 2, 2016 at 16:30
It is iterating through all the items in path, passing the current item being visited as x and comparing to the next in path (as path[-1]) as to get the min distance. — SerialDev
– SerialDev, Commented Dec 2, 2016 at 16:34

Kirill Bulygin · Accepted Answer · 2016-12-02 17:02:07Z

1

closest becomes to_visit[i] such that

distance(path[-1], to_visit[i]) = 
  min(distance(path[-1], to_visit[0]), distance(path[-1], to_visit[1]), ...)

In other words, the lambda function makes comparison not by to_visit[i] but by distance(path[-1], to_visit[i]).

edited Dec 2, 2016 at 17:02

answered Dec 2, 2016 at 16:29

Kirill Bulygin

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2 Comments

vaultah Over a year ago

min returns the x element from to_visit having the lowest distance(path[-1], x), not the distance itself

Kirill Bulygin Over a year ago

Yes, you are right.

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